Latexes are widely used in the coatings industry because of their excellent dry characteristics, low VOCs and the ease of application to various substrates. These qualities are important for architectural and industrial applications. Latex properties can be improved by the addition of a urethane linkage into the latex. For example, chemical resistance, hardness, toughness, and dirt pick-up resistance can be improved by the addition of a urethane linkage to latex.
U.S. Pat. No. 6,153,690 describes a method for forming a latex comprising a urethane linkage by first forming a latex comprising an isocyanate reactive monomer by emulsion polymerization and then introducing an isocyanate to the previously formed latex in order to form a urethane linkage between the isocyanate reactive monomer and the isocyanate. In U.S. Pat. No. 6,153,690, hydrophobic isocyanate is present as a relatively large, unstable oil droplet in aqueous phase without surfactant.
There continues to be a need for urethane linkage-containing latex having improved properties. Exemplary desired improved properties include higher solids content, higher solids content in combination with lower viscosity, and higher solids content in combination with low coagulum content.
A latex with high solids content produces a coating that is more desirable from a performance standpoint. A coating with a high solids content is more efficient and economical because it is possible to achieve a desired coating thickness on a substrate with less coating (i.e., fewer layers of a coating with a high solids content are needed to achieve the desired coating thickness). However, during the polymerization, the presence of unstable oil droplet as in U.S. Pat. No. 6,153,690 results in the formation of high levels of coagulum which has to be removed as waste material. As a result, solids content is reduced accordingly due to the loss of usable polymer content in the emulsion product.
Further, higher solids content and low viscosity generally are desirable for greater ease of manufacture and cheaper transportation costs for the latex polymer. For example, a higher solids content provides more flexibility for coating formulators. A higher solids content also allows less water to be shipped to coating formulators.
In addition, an improved process that produces more usable product rather than production waste is desirable. Coagulum is detrimental in latex production processes because it accumulates in the processing equipment and is discarded as waste. Once a process hindering amount of coagulum accumulates in the processing equipment, the equipment is cleaned to remove the coagulum. The cleaning process is costly because of the cost of cleaning and also because of processing downtime associated with cleaning. Further, coagulum is considered to be production waste that is discarded.
There is a need for a process that produces urethane-containing latexes having relatively higher solids content, relatively lower viscosity, and that produces more usable product and less coagulum.